Make arch thicker but retain diameter?
Start by placing the 3D cursor exactly at the center of the arch.

Select the outer (or inner) faces only of the cyclinder only.

Press . (Period key) to transform relative to the 3D Cursor position.

Then press S to scale and Shift+Y to exclude the Y axis (adjust according to your own orientation). Now adjust the radius as desired.



Ideally though, if you want to do it precisely (with accurate measurements) and in a non destructive manner, make your arch paper thin by removing either the inner or outer faces, then use a Solidify modifier instead to add thickness.